Hi Muhackl, on the face of it you have a 60 watt power supply (19v x 3.42A) feeding an amplifier rated at 2 x 25 watts output power, even with 100% efficiency this leaves only 10 watts (2A at 5v) to feed the pi, the screen and the hard drive. The hard drive will take about 1A at start up, leaving 1A to power screen and pi, at a time (ie boot up) when the pi is busy and taking maximum juice, too! A higher rated power supply might help but the 5v regulator built in to the amp may be a limiting factor, 2A at 5v was good for a pi supply when the amp was was designed, once you start stressing it to the limit you lose regulation and the voltage will drop anyway. You need to check with Hifiberry support what current it is designed to supply.
If you intend to build it all into a box there are plenty of cheap voltage regulators you could use to power the screen and/or the hard drive seperately whilst still retaining a single external feed. Try http://www.hobbytronics.co.uk/batteries.
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